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NCHRP Project 17-35: Evaluation of Safety Strategies at Signalized Intersections

 

 

Evaluations of Low Cost Safety Improvements Pooled Fund Study

 

PPT version for Printing

Flashing Beacons at Stop Controlled Intersections Results

Header image – Picture shows series of three scenarios: a vehicle on a meandering road, safety personnel at work, and a car that is very badly damaged after it appears to have collided into a telephone pole.

Dr. Raghavan Srinivasan, HSRC


Background on Strategy

  • Alert drivers to the presence of an intersection
  • Reduce angle crashes
  • Initial installation costs: $2,000 to $ 100,000
  • Placement Options
    • STOP Sign mounted
    • Overhead
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Literature Review

  • Previous Work
    • Cribbons and Walton 1970
      • 14 rural sites
      • About 1 year before, 1 year after
    • Pant et al. 1999
      • 6 sites without and 7 with a beacon
    • Murphy and Hummer 2007
      • 34 locations in North Carolina
      • Empirical Bayes study – accounted for change in traffic volume using a linear assumption
  • Safety effectiveness has not been adequately quantified

Objective

  • To examine the safety impact
  • Target crash types considered included:
    • Total intersection crashes.
    • Total intersection injury and fatal crashes (including fatality (K), Injury, incapacitating (A), Injury, non–capacitating (B), and possible injury (C).
    • Total intersection angle crashes.
    • Total intersection rear–end crashes.
  • Determine if safety impacts are function of:
    • Area type (rural, suburban, or urban).
    • Intersection type (Two–way versus four– way stop–controlled).
    • Types of flashing beacon installations including standard and actuated.
    • Location of the beacon including stop sign or overhead.

Study Design

  • Based on methodology suggested by Hauer (1997)
  • Literature review to determine the average number of crashes at stop controlled intersections
  • Sample size
    • Minimum: 53 intersection–years in the before period (ability to detect 20 percent reduction in crashes at 90 percent confidence level)
    • Desirable: 260 intersection–years in the before period (ability to detect 10 percent reduction in crashes at 90 percent confidence level)

Data Collection

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Data Collection – North Carolina

Total number of intersections used = 64

Variable Before After
Site–Years 583 305
Major Road AADT 3578 5105
Minor Road AADT 1540 2074
Total Crashes per site–year 2.85 2.99
Angle Crashes per site–year 1.66 1.45
Injury and Fatal Crashes per site–year 1.68 1.58
Rear–end crashes per site–year 0.31 0.42
 

Data Collection – South Carolina

Total number of intersections used = 42

Variable Before After
Site–Years 334 128
Major Road AADT 3978 4531
Minor Road AADT 1938 2192
Total Crashes per site–year 2.73 2.64
Angle Crashes per site–year 1.17 1.27
Injury and Fatal Crashes per site–year 0.94 0.89
Rear–end crashes per site–year 0.55 0.61
 

Aggregate Evaluation Results: North Carolina

  Angle Injury and Fatal Rear–end Total
Expected crashes without strategy 532.6 533.7 148.0 973.2
Observed crashes 436 469 127 894
Estimate of percent reduction 18.3 12.2 14.6 8.2
(standard error) (4.9) (5.1) (9.7) (4.0)
 

Aggregate Evaluation Results: South Carolina

  Angle Injury and Fatal Rear–end Total
Expected crashes without strategy 156.6 115.1 73.6 323.8
Observed crashes 162 114 78 338
Estimate of percent reduction –2.7 1.8 –3.9 –4.0
(standard error) (11.7) (12.9) (18.5) (8.2)
 

Aggregate Evaluation Results: NC and SC Combined

  Angle Injury and Fatal Rear–end Total
Expected crashes without strategy 689.2 648.8 221.6 1297.0
Observed crashes 598 583 205 1232
Estimate of percent reduction 13.3 10.2 7.9 5.1
(standard error) (4.6) (4.8) (8.9) (3.6)
 

Disaggregate Evaluation Results: Angle Crashes

Disaggregate Group Sites Estimate of percent reduction (Standard Error)
Rural (NC and SC) 76 15.7 (5.3)
Suburban (NC) 14 11.8 (10.2)
Urban (NC and SC) 16 –12.3 (23.4)
2–way stop (NC and SC) 95 12.7 (4.7)
2–way stop (SC) 31 –10.4 (13.4)
4–way stop (SC) 11 27.8 (20.5)
Beacon Type–Standard (NC and SC) 89 13.3 (5.2)
Beacon Type–Standard (NC) 47 19.8 (5.7)
Beacon Type–Other (NC) 17 14.0 (9.8)
Beacon Location–Overhead (NC and SC) 97 14.7 (4.8)
Beacon Location–Other (NC and SC) 9 –5.8 (18.4)
Beacon Location–Overhead (NC and SC) 97 14.7 (4.8)
 

Economic Analysis

  • FHWA cost per crash for unsignalized intersections (FHWA–HRT–05–051)
    • $13,238 for rear–end
    • $61,114 for right–angle
  • Economic Benefit
    • Based on combined results from NC and SC
    • $ 12,040 per site–year
  • Costs estimated for Standard and ‘Other’ types separately
  • Standard Beacons:
    • $ 27,500 initial installation costs (high value)
    • Annual Maintenance and other costs: $ 720
    • 10 year life
    • Annualized costs = $ 4,636
    • Benefit Cost ratio is 2.6:1
  • ‘Other’ Beacons:
    • Initial costs range from $ 5,000 to $ 100,000
    • Benefit achieved if initial cost is less than $ 79,000
    • 2:1 Benefit/Cost ratio if initial cost is less than $ 37,000

Conclusions

  • NC: statistically significant reduction in total, angle, and injury and fatal crashes
  • SC: very little change
  • NC and SC combined: statistically significant reduction in angle and injury and fatal crashes
  • Probably more effective in rural and suburban locations
  • Very few locations with stop sign mounted beacons
  • Standard beacons provide a benefit cost ratio of 2.6:1
  • Net benefit of non–standard beacons depends on their cost

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